I'm still throwing money into an R100RS I picked up for $1000. It had been stored under a tarp in the guy's backyard for 8 years without running. So far:
-rebuilt the carbs
-opened up and cleaned the transmission and final drive
-replaced the gas tank
-powdercoated frame bits
-replaced the charging system.
Still to do:
At the end of the day, I'll have spent what a running bike would have cost me and I'll know it inside and out. You're about to spend a whole lot more on a bike worth a whole lot less. Go back and find one good part on that bike. Buy that one part and bring it home. Then start looking for all the parts you'll need to build a running bike to add to it and you won't have to deal with a bunch of junk. I'm thinking $10 for the handlebars and go from there.